Rock Basins are a rather cool erosion phenonema that are always intriguing when found. A shallow depression in an explosed granite boulder, perhaps started by the presence of a weeker feldspar phenocryst, is gradually attacked further and deeper by a persitent shallow pool of water. Once the frost and the wind get involved by eddying the water around in the basin scoop then the cycle gets ever more pronounced.
Rock Basins sometimes end up drilling themselves right through the side wall, or even more impressively, through the bottom of a flat boulder, producing an impressively sculpted piece of stone that have been linked to druid ceremonies.
The Rock Basin here was found on the upper outcrop of Hucken Tor SX 551739, although the lower crags also have further examples.
The Sundew plant Drosera Rotundifolia feeds on insects that have been attracted to the glistening drops of mucilage on its spiney red leaves. The droplets covering the leaves are loaded with a sugary substance however the stickiness is overwhelming for the small insects it attracts. Enzymes dissolve the insects in situ and nutrients are obsorbed into the foliage.
Sundew have developed their carnivorous behaviour to substitute for the lack of nutrients of their chosen environment, namely damp acidic soil, marshes and blanket bogs. The insects provide the necessary nitrogen and other nutrients missing from such soils.
Sundew – Drosera Rotundifolia
This example was found close to a tributary of the river Walkham on South Dartmoor. This section often floods leaving the area squelchy in all but the driest summers.
Early tin streamer worked their tin ore on site. They would crush the tin-baring gravel in a block of granite hollowed out and used as a mortar, in much the same way as a mortar and pestle are used in the kitchens.
Occassionally you can still find discarded mortarstones in old tin-streaming areas. The best places to find them are nearly always near to an old tinners hut within a tin-streaming valley.
This example was found at Plym Steps tucked into the hillside by an old tinners hut.
As Dartmoors plutons pulsed and pushed up and through the country rocks of Devon 270 million years ago, some of these metamorphosed sedimentary layers above ,shattered and sank their altered and fractured boulders into the crystal mush of the rising and cooling magma. As they sink they are generally chemically altered, abraded and rounded off by the intense heat and continued pressure of this new super-hot environment. These resultant buried rocks and boulders are permenantly embedded into the granite framework until exposed later within the walls of a Dartmoor granite tor. These bizarre and erratic stones are more commonly known as Xenoliths.
By definition a Xenoliths, meaning ‘foreign stones’.
Dartmoor is one very large pluton of granite however it’s development 270 million years ago has led to some variety to its colours, forms and crystal makeup. Here we have some well developed Feldspar Phenocrysts in a granite mass. This strangley clean surface was found at Calveslake Tor. Each of the white Phenocrysts are approximately 10cm across.
By definition a Phenocryst is; an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock.